The use of membrane cells for electrolytic decomposition of brine to produce sodium hydroxide, chlorine and hydrogen is now the major process employed in new and converted plants because it provides many advantages over the older electrolytic processes, which required the use of mercury or deposited diaphragms. Among these advantages are elimination of mercury and asbestos pollution problems, reduced power consumption, improved cell efficiency, and better quality of chlorine and caustic.
The use of membrane cells requires salt of high purity with minimum impurities such as calcium, magnesium, barium, strontium, and other metallic impurities in the aqueous sodium chloride brine fed to the cells because the membranes are easily plugged or otherwise fouled. For example, one part or more of calcium ions per million parts of brine could be tolerated in the diaphragm type cells. This amount of calcium in the feed brine to membrane cells will damage the membranes severely and will reduce the membrane life from 3 to 5 years to less than 6 months. The high calcium causes a major drop in cell efficiency with a corresponding increase in power consumption resulting in shortened membrane life. Also, other metallic impurities, such as barium, strontium, magnesium, nickel, iron, copper, titanium, aluminum and other multivalent metals are detrimental in the brine being electrolyzed in membrane cells and must be removed or minimized. Furthermore, most membrane cells require a sodium sulfate level of less than 8 grams per liter in the feed brine to the cells to avoid lower efficiencies, including higher power costs, damage to the anode coatings, and damage to the membranes.
In the operation of diaphragm cells, the major operating cost factors are power, steam, and brine. Membrane cells offer significantly reduced power and steam costs but brine (salt) costs are higher than alternative cells because the membranes require high purity brine to operate efficiently. This necessitates removal of the buildup of certain impurities in the system, such as those referred to above, which are harmful to the performance of the membrane cells.
The source of sodium chloride used in membrane cells may be rock salt, solar salt, byproduct salt from an existing diaphragm cell plant, also known as caustic process or C.P. salt, purified evaporative salt, or brine delivered in a pipeline which has been solution mined from underground salt deposits. Nearly saturated sodium chloride brine from any source contains roughly three pounds of water per pound of sodium chloride. This invention relates to the process involving feeding the chlorine/caustic plant with purified evaporative salt.